1. Field of the Invention
This invention relates generally to a load driving circuit device, and more specifically, to a load driving circuit device that can reduce common mode noise.
2. Description of the Related Art
FIG. 7 is a simplified circuit schematic representation of a conventional load driving circuit device. As shown in this figure, load driving circuit device 101 and electrical load 130, such as a motor, solenoid, or lamp, are connected by two parallel wiring harnesses 120a and 120b. One of wiring harnesses 120a is connected to a terminal 141 on the high potential side of a direct current power supply. The other wiring harness 120b is connected to the low potential side of the direct current power supply—that is, ground—via a switching means 113 of load driving circuit device 101. Power supplied to the electrical load is pulse width modulation controlled by turning on and off switching means 113 with a high frequency.
The potential of wiring harnesses 120a with respect to ground is fixed at the potential of the high potential side of the direct current power supply. In contrast, the potential of the other wiring harness 120b changes repeatedly between ground potential and the power supply voltage according to on/off of switching means 113. For this reason, the average potential of the two wiring harnesses 120a and 120b with respect to ground potential—that is, common mode potential Vcom—also changes repeatedly between “½×power supply voltage” and “power supply voltage.” It is known that common mode noise is generated as a result.
There is a need, therefore, for a circuit arrangement that will drive a load, particularly an inductive load, while suppressing common mode noise.
The prior art has sought to alleviate the problem of common mode noise by causing the potential of the wiring harness on the positive electrode side and the potential of the wiring harness on the negative electrode side to fluctuate symmetrically. As a result, the common mode potential is fixed, and its fluctuation is suppressed. In this manner, common mode current is reduced and the generation of common mode noise is suppressed. However, the known arrangement requires that a transformer be arranged on the output side of the load driving circuit device. Thus, both the size and the cost of the load driving circuit device are disadvantageously increased. In the case of a large load current, the transformer must also be large, and as a result, such problems become particularly troublesome.
It is, therefore, an object of this invention to provide a small, low-cost load driving circuit device that reduces common mode noise without requiring the use of a transformer.